First, the gripping case we've been tracking. The utah doctor on trial for murdering his wife. And yesterday, the most emotional testimony yet when the doctor's oldest daughter took the stand to testify against him. Abc's aditi roy has all the latest. Do you recognize this man sitting here? Yes. With the blue tie? Yes. Who is he? My father. Reporter: Rachel macneill began her tearful testimony against her own father. Speaking barely above a whisper. Growing up, my father was my best friend. Reporter: Now, she believes the man sitting across the court from her thursday, killed her mother. He said he was concerned that there would be a police investigation. He didn't want anyone to think that he murdered my mother. It was so shocking to me. I said, why? Why would anybody think that? Reporter: Martin macneill appeared emotional during his eldest daughter's testimony. He insists he's innocent. His attorneys say michele MacNEILL DIED FROM A HEART Disorder in april of 2007. But prosecutors say the former utah physician gave his wife a fatal dose of painkillers, while she was recovering from a face-lift. One they say he insisted she have. The alleged motive, an affair. Rachel told jurors her father's purported mistress became their live-in nanny shortly after their mother's death. She was sitting there. The children were taking care of themselves. Reporter: During CROSS-EXAMINATION, MacNEILL'S Attorney tried to point out inconsistencies in her testimony. You never said anything about your dad joking about being single at the luncheon, did you? I don't recall. Reporter: And questioned her mental health. And you've been diagnosed bipolar? Have I in my lifetime been diagnosed as bipolar? Yes. Reporter: Rachel's sister, alexis, testified in a closed jury hearing whether the youngest MacNeill, ada, should take the stand. She was 6 when she discovered her mother unconscious in the tub. I've never allowed her to be interviewed. I never allowed her to watch any of the media, television, newspaper. I was concerned. I didn't want her to be tainted in any way. Reporter: Today, the judge will decide whether ada will be able to tell the jury what she saw the night her family was torn apart. For "good morning america," aditi roy, abc news, los angeles. Aditi for that. Let's bring in chief legal affairs anchor, dan abrams. Pretty devastating testimony. Oh, yeah. You have this woman talking about everything her father did and said immediately after her mother died, which leads to all these questions. Meaning, this daughter didn't suspect her father. The other daughter did. This daughter, when this happens, is thinking, there's no way that my father could have been involved. And so, when he starts talking about things like, well, we need to get an autopsy immediately. She pointed out that he gave her $5,000 in cash, which he had never given her before. And she's thinking, why is he doing this, et cetera. Very powerful. His lawyers, they didn't give an inch on cross-examination. They have to. If you view her as a really important witness, which she is, they have to go after her. From their perspective, she's lying, right? From their perspective, she's not giving a truthful account. They had to go after her. But it creates an awkward situation, where you have this woman, who seems so emotional. And his lawyers have to basically say, you're not telling the truth and, "b," you have a history of mental illness. And the judge has to make a big decision, whether or not to let the 12-year-old daughter, ada, to testify about when she found her mother's body. When she was 6. The judge is going to let in the testimony. There's going to be restrictions. But it's pretty clear the judge is going to allow it. How do you cross-examine a little girl? They have to do it with kid gloves, so to speak. Gingerly. They have to be tender. They can't go after her. And I think they're going to have to call in other witnesses, primarily, to say she's got -- there could be memory problems here. Memory experts. Things someone this age, et cetera. They don't want to push her too much. Dan abrams, thanks very much.

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